Why dewaxing is critical
Wax expands several times more than ceramic when heated. If a wax-filled shell is heated slowly, the bulk wax expands before it can escape and the shell is stressed in tension until it cracks. The remedy is very rapid surface heating: a thin layer of wax at the shell interface melts first and drains or soaks into the shell, creating the clearance into which the remaining wax can expand.
Dewaxing technologies
- Steam autoclave. The standard industrial method. The vessel is pressurised with saturated steam within a few seconds, typically to several bar, so that heat transfer to the shell surface is extremely fast. Most of the wax is recovered and reconditioned for reuse.
- Flash-fire dewaxing. Shells are placed directly in a furnace at high temperature. Wax melts and partly burns; recovery is lower and emissions must be treated, but dewaxing and firing are combined.
- Other methods. Hot liquid and microwave-assisted dewaxing are used in particular cases.
Shell design also helps: wax vents in heavy sections, pattern waxes with suitable melting behaviour and adequate shell green strength reduce cracking. Fine cracks that do occur can sometimes be repaired with ceramic patching before firing; larger ones produce fins on the casting.
Printed and polymer patterns
Patterns that do not melt like wax, such as many printed resins and polystyrene, cannot be autoclaved in the same way. They are burnt out in the furnace, which requires thin-walled or internally structured patterns, sufficient oxygen and longer burnout cycles to avoid shell cracking and ash residues.
Firing
Firing has three functions: it burns out residual wax and organic matter, it sinters the binder so that the shell develops its fired strength, and it brings the shell to the temperature at which it will be poured. For steels, shells are commonly fired and poured at roughly 900 to 1100 °C; aluminium is poured into considerably cooler shells. Soak time must be long enough for the full shell thickness and any ceramic cores to reach temperature and for carbon residues to oxidise completely.
Preheat temperature as a process parameter
- A hotter shell improves filling of thin walls and reproduction of detail, but slows solidification, coarsens the grain, promotes reactions between metal and face coat and can increase shrinkage defects.
- A cooler shell refines the structure and shortens solidification, but increases the risk of misruns and cold shuts.
Shells lose temperature quickly once removed from the furnace, so the time between furnace and pouring is controlled. For parts with very different wall thicknesses, insulating wraps are sometimes applied to selected areas to steer solidification.
Related: melting and pouring.